A unitized fiber optic cable is a cable that contains a number of unit or subunit cables. Each of these unit or subunit cables contains multiple optical fibers, typically surrounded by a unit cable jacket. One reason for separating optical fibers into separate units within a unitized cable is to simplify access to individual optical fibers and to aid in identifying individual optical fibers. Higher fiber count premises cables generally comprise unitized cables including individual unit cables. These prior art unit premises cables typically contain six, twelve or more tight buffered optical fibers, having a tight buffer layer or coating forming part of the individual optical fibers, and tensile strength members, such as aramid fibers, within an unit cable jacket. Typically aramid fibers are located between the tight buffered optical fibers and the unit cable jacket, where the aramid fibers will keep the tight buffered optical fibers from adhering or sticking to the unit cable jacket. Sticking could otherwise occur if a unit cable jacket is extruded around tight buffered fibers, containing the same resin that is used in the unit cable jacket. In prior art high fiber count premises cables, the unit jacket as well as the tensile strength members are relied upon to protect the optical fibers from damage. As a result, the thickness of prior art unit jacket is on the order of 0.5 mm and the unit jackets typically contain a polymer, such as polyvinyl chloride having a concentration of between 50% and 75%. Prior art unit cables can also include a glass reinforced plastic (GRP) antibuckling member, typically located at the center of the optical fibers. Prior art unit cables also typically include a ripcord.
In these prior art unitized cables, several unit cables are bundled or stranded together within the outer unitized cable jacket to form a single high fiber count premises cable. This prior art outer unitized cable jacket does not significantly protect constituent optical fibers from tensile or compressive loads. Each individual unit cable instead includes tensile and compressive load bearing members.
Examples of unitized fiber optic cables can be found in U.S. Pat. No. 6,249,628 and in European Patent Application EP 1 041 420 A1. The unitized fiber optic cables disclosed therein each include a plurality of unit or subunit cables in which a layer of dielectric strength members surrounds individual components in each unit cable. These dielectric strength members surround cables that can include at least two optical fibers in turn surrounded by at least one generally round buffer tube. Each optical fiber has a glass core, and inner and outer primary coatings of a thermoplastic that is surrounded by a coloring layer. A tight buffering coating over the coloring layer is not altogether necessary, but may be included in some applications. However a generally round buffer tube with a nominal thickness of 0.170 mm and a maximum wall thickness of 0.240 mm surrounds the two optical fibers. These buffer tubes shown in the prior art are examples of loose buffer tubes which are not bonded to the optical fibers, but which can be extruded around the glass fiber and are employed for fiber protection and segregation. In tight buffered cables, a protective thermoplastic coating is extruded directly on the individual glass fibers.
FIGS. 1 and 2 also show specific examples of prior art unit cables that can be incorporated into a prior art unitized cable such as that shown in FIG. 3. FIG. 1 shows a prior art single layer unit cable 100 in which a plurality of tight buffered optical fibers 30 surround a glass reinforced plastic (GRP) member 112, which can have a thermoplastic outer layer 110. This central strength member 110 can serve as an antibuckling member. A layer of aramid strength fibers 108 surrounds the optical fibers 30 and separates the optical fibers 30 from an unit jacket 106 that can be extruded from a thermoplastic resin, such as polyvinyl chloride. The unit jacket 106 also functions as a load-bearing member to protect the optical fibers 30 from damage. A ripcord (not shown) can also be embedded between the aramid fibers 108 and the unit jacket 106 and can be used to sever the unit jacket 106 to access the optical fibers 30 when necessary. Typically the prior art single-layer unit cable 100 shown in FIG. 1 has a nominal outer diameter of 6.6 mm and can include twelve individual optical fibers 30.
FIG. 2 shows a prior art dual-layer unit cable 102 in which twelve optical fibers 30 are arrayed in two generally concentric layers. Aramid yarns 108 separate the two concentric layers and surround the outer layer. Aramid yams 108 can also be positioned at the center of the optical fiber array. As with the prior art single layer unit 100, a unit jacket 106, formed primarily of a material such as polylvinyl chloride, surrounds the outer aramid layer 108. This prior art dual-layer unit cable 102 can have an outer diameter of 5.7 mm. In both the single layer and dual layer unit cables, the unit jacket 106 can have a thickness of approximately 0.5 mm and serves not only to segregate optical fibers 30 into identifiable unit cables, but also provides physical protection for the optical fibers.
FIG. 3 is an example of a prior art unitized cable 104 including twelve of the dual-layer unit cables 102 shown in FIG. 2. The unit cables 102 are arrayed in two layers and surrounded by an outer cable jacket 118. The nominal diameter of this prior art unitized cable is 23.9 mm. Since this unitized cable 104 includes the smaller dual layer unit cables of FIG. 2, a unitized cable containing prior art single layer unit cables of the type shown in FIG. 1 would be expected to have a larger outer diameter.
The outer diameter of these prior art premises cables is significant because space is often limited in locations, such as risers and trays in which multiple unitized cables are to be located or at entrances and exits to wiring closets and other similar locations. Therefore a reduction in the diameter of unitized cables, without compromising their physical integrity is desirable. Use of the instant invention results in unit cables and unitized cables having a smaller nominal outside diameter and potentially a smaller size in actual practice.